4 sticks (1 pound) unsalted butter, at room temperature
8 cups (2 pounds) confectioners' sugar
1 tablespoon pure vanilla extract
1 to 2 tablespoons milk
Assorted gel food coloring
Three 8-inch vanilla cakes
Combine the butter and confectioners' sugar in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until mostly incorporated. Add the vanilla and mix on medium-high speed until smooth, light and fluffy, about 3 minutes. Adjust the consistency with the milk, if needed, mixing in 1 tablespoon at a time until spreadable.
Remove 1 cup of the buttercream from the bowl and divide it among individual small bowls--1 for each color in your floral palette. (Keep the remaining buttercream in the stand mixer bowl white and set aside for frosting the cake.) Keep the buttercream in 1 small bowl white and tint each of the remaining buttercream with your desired gel food coloring. It's best to tint the buttercream in a few different shades (light, dark and darker).
To frost the cake: Place 1 of the cakes on a cake stand and spread 1 cup of the white frosting on top. Repeat to make 3 layers, ending with the third cake. Cover the top and sides with a thin layer of the white frosting (this is the "crumb coat;" it doesn't have to be perfect). Refrigerate until the crumb coat is firm, about 20 minutes. Spread the remaining white frosting on the top and sides of the cake, smoothing it out as you go. Refrigerate until firm, about 20 minutes.
To paint the flowers: Start with the darkest buttercream and paint the shape of a flower petal with a small offset spatula or a palette knife. (It might look like a colored circle but it will all come together when you add the other colors).
Next, add the white and lighter shades of buttercream on top of the petal using a swiping motion (like you are making the arches of a heart) and go around the shape of the petal. Paint larger and smaller flowers around the main flower using the white and darker shades of buttercream to create depth. Repeat these steps all around the cake and decorate, as desired.